This invention relates to circuit encapsulation.
The circuit 10, shown in FIGS. 1a and 1b, for example, has integrated circuit dies (i.e., semiconductor dies) 12, and 14, and other electrical components 15, 16, 17, and 18 connected by a printed circuit board (PCB) 20. Encapsulation of circuit 10 is done within a mold cavity 22 of a mold 24 (FIGS. 2a-2e) using a molding compound 35. Connections to PCB 20 are made available externally to the molding compound by soldering conductive leads 26a, 26b, 26c, 26d, 26e, and 26f (FIGS. 1a, 1b) of metal lead frame 26 to input/output (I/O) pads 30 of PCB 20.
I/O pads 30 are also connected through conductive leads to the electrical components of circuit 10. For clarity, only conductive leads 28 (FIG. 1a) and 29 (FIG. 1b) are shown. Metal lead 26bis connected to an I/O pad 30 which is connected to conductive lead 28. A conductive pad 12a on semiconductor die 12 may be connected through wire 12b to conductive lead 28 (FIG. 2b) or a conductive pad (not shown) on a bottom side of die 12 could be connected to conductive lead 28. Metal lead 26f is connected to an I/O pad 30 which is connected by a plated through hole 31 (FIG. 2C) to conductive lead 29. A conductive pad (not shown) on a bottom side of electrical component 17 is directly connected to conductive lead 29.
Mold 24 has a top 24a and a bottom 24b that are closed on portions 26g and 26h (FIGS. 2a, 2d), and leads 26a, 26b, 26c, 26d, 26e and 26f (FIGS. 2b, 2c, 2e) of lead frame 26. Once mold 24 is closed, transfer molding is done by pushing molding compound 35, at 1000 psi, from a pot 36 (in mold bottom 24b), using a piston 40, into a runner 38 (also in mold bottom 24b) and into mold cavity 22 to surround circuit 10. After molding, circuit 10, encapsulated in cured molding compound, is removed from mold 24, and lead frame 26 is cut along dashed line 42 (FIGS. 1a, 1b), and waste molding compound is trimmed away along dashed line 43.
In one example, lead frame 26 has a thickness of approximately 0.008 inches with a tolerance of +/-0.00025 inches. When lead frame 26 is at a minimum thickness (i.e., 0.008-0.00025=0.00775), top 24a closes against bottom 24b, as shown in FIGS. 2a, 2b, 2c (i.e., they touch). The pressure of mold 24 on lead frame 26 does not damage lead frame 26. When lead frame 26 is at a maximum thickness (i.e., 0.008+0.00025=0.00825), top 24a does not close against bottom 24b leaving a gap 44, as shown in FIGS. 2d, 2e. The maximum height of gap 44 is 0.0005 inches (i.e., maximum thickness variance, 0.00025+0.00025=0.0005). Typical molding compounds will not leak, indicated by arrow 45, through a gap 44 of 0.0005 inches or less due to the viscosity of the molding compound.
Referring to FIGS. 3a, 3b, 3c, metal lead frame 26 (FIGS. 1a, 1b) is not needed to make external connections to I/O pads 30 of a component 9 because only a portion of one side of PCB 20 is encapsulated in cured molding compound 50 leaving I/O pads 30 exposed. Electrical components are generally not mounted in the exposed areas.
The thickness of PCB 20 is approximately 0.020 inches and has a tolerance of +/-0.0025 inches. The maximum thickness variation of 0.005 inches (i.e., 0.0025+0.0025=0.005) makes it difficult to use conventional molds to fully encapsulate circuit 10 (i.e., top, bottom, and sides) with the exception of I/O pads 30 (component 9, FIGS. 4, 5a, and 5b).